With his destination in clearer view than ever, Jason Castro’s ride in the fast lane toward the major leagues had hit an unexpected speed bump.

The consensus top position prospect in the organization for most of his time in the organization, Castro lost the spring training battle to J.R. Towles for the starting job in Houston, and then upon being assigned to Class AAA Round Rock, went through the first prolonged struggles of his career.

Tonight, with his disappointing April far in the rearview mirror, Castro will make the most eagerly anticipated major league debut for the Astros since Hunter Pence in April 2007.

So how big was the change that turned his season around and allowed him to reach the Astros before the

All-Star break?

Barely noticeable, even from the expensive seats.

“I made a little adjustment in my swing, and it really helped me get real efficient with the fastball,” Castro said.

The change was in his back elbow, which aided his approach to the fastball on the inner half of the plate. “He was much quicker to the ball and is now able to drive the ball to all parts of the field,” assistant general manager for player development Ricky Bennett said in an e-mail.

April power shortage

As a whole, Castro’s path to the big leagues, despite the poor showing in April, has been relatively smooth and free of injuries.

Drafted out of Stanford with the Astros’ first pick in the 2008 draft (No. 10 overall) Castro immediately replaced Towles, who struggled mightily after a sensational 2007 call-up, as the mythical “catcher of the future.”

Castro debuted with a productive 39 games in short-season Class A. In 2009, he skipped regular Class A, made easy work of high Class A Lancaster and jumped at midseason to Class AA Corpus Christi, where he hit for high average but saw his power numbers drop away from the hitter’s haven of Lancaster.

In early 2010, following a stint with the big club as a non-roster invitee to spring training, most of his offensive tools seemed to disappear.

He hit .226 in April, and while he was supplementing it with excellent walk totals, there was a scary absence of power — one double, no triples and no home runs in 53 at-bats and a .245 slugging percentage.

Finds groove in May

The average came back in May, with Castro hitting .293 thanks in part to the adjustment. The power returned in June with three home runs in 15 games, including one in what could have been his last game with Round Rock on Saturday night before getting called up along with third baseman Chris Johnson and outfielder Jason Bourgeois.

“Jason’s hitting approach has been very good from the start,” Bennett said. “He just needed the experience and ABs before he was ready for the major leagues.”

First-hand and second-hand Accounts from scouts regarding the home run power Castro will provide at the major league level have trended a bit downward since he entered the organization. Still, he should provide fairly strong power to the gaps with his lefthanded swing.

Through all the changes in his offense, Castro’s catching has remained a strong phase of his game. “The feedback that we’ve gotten all along is that from a defensive standpoint, from a game-calling standpoint, that he was major league ready,” general manager Ed Wade said. “This kid’s very bright with a great skill set, and we’re excited to have him.”

Astros reliever Wesley Wright, who threw to Castro in eight of his nine starts for Round Rock this season, praised the rookie.

“He’s a guy that’s going to be a great catch-and-throw guy,” Wright said. “He receives the ball well, very agile back there and has a great arm.”

Castro said after his call-up that he was excited to build on the relationships he had in spring training in terms of calling a game, and according to Wright, Castro will let the pitcher control a lot of that relationship. “He’s going to get a feel for the guys and see what they like and don’t like and go from there,” Wright said. “I don’t think it will be a tough transition. I think it’s a situation where once he gets here and gets used to everybody, he’ll pick it up pretty good.”

The buzz over Castro’s arrival is being felt at the top.

“We’ve got high hopes for him, and he has really made progress at the AAA level,” owner Drayton McLane said. “We think he has lots of potential.